Laminar flow fans are commonly used for impelling a fluid medium (a liquid or gas). In particular, they are suited to low flow, high back pressure forced air cooling of electrical apparatus when low acoustic fan noise is required. Typically, these fans comprise an assembly of parallel annular or frusto-conical elements, mounted spaced apart and face to face about a common rotation axis. In operation, the assembly is rotated about this axis by a suitable motor; the fluid is drawn in at the center of the assembly, impelled towards the outer edges of the disks by viscous interaction with the surface of the disks, and is finally expelled at the periphery of the disk assembly.
The fan elements are usually held together by bolts or rods passing through the elements. At least three bolts, angularly spaced around the assembly, are used, in order to provide enough points of support to maintain the elements parallel to one another. During manufacture, one element is placed onto the mounting bolts, followed by a number of spacers (one for each bolt), and then by the next element and so on. When all of the elements and spacers are in place, the assembly is held together by fastening a corresponding nut to each of the mounting bolts.
The above method can be time consuming and expensive. Several intricate process steps are required to place each element and set of spacers into position, and a large number of small components are required.
A development is described in GB 2126653, in which the first step is temporarily to hold the fan elements in the required spaced apart relationship using a suitable jig. A plurality of holes are provided in each element; these are aligned with the holes in other elements when the elements are held in the jig. Spirally fluted tubes or rods are then inserted through the holes, passing through each element in turn. The fluted tubes are arranged to grip the periphery of the holes and so to hold the disks rigidly in the required relationship. In this arrangement, no spacers are required.
A problem with both of the above methods is that the mounting bolts, tubes or rods pass through the `active area` of the fan elements. This can cause turbulence in the air flow, which can reduce the efficiency of the fan and increase the fan noise. The turbulence problem is exacerbated when spacers are used between the fan elements.
It is a constant aim to decrease manufacturing costs and to reduce the number of separate components required during a manufacturing process.